1. Field of the Invention
The present invention relates generally to a motor-driven power steering system for automotive vehicles, and more particularly to a motor-driven power steering system for automotive vehicles which employs an electric motor for generating assistive steering torque.
2. Description of the Relevant Art
There have been proposed in recent years various motor-driven power steering systems for automotive vehicles which employ an electric motor for generating assistive steering torque.
One known example is a steering system disclosed in U.S. Pat. No. 4,530,413 patented July 23, 1985. The disclosed steering system comprises a torque detecting mechanism for detecting steering torque Ts applied to an input shaft coupled to a steering wheel, an output shaft coupled to a gear box connected to steerable wheels, a DC motor for exerting assistive steering torque to the output shaft, and a control circuit for controlling the operation of the motor based on a signal from the torque detecting mechanism. The control circuit includes an anti-log power amplifier for exponentially modifying a steering torque input signal applied thereto, and applies an armature voltage proportional to the output of the anti-log power amplifier to the motor. Any vehicle speed sensor which would be required in ordinary motor-driven power steering systems is dispensed with by the use of the anti-log power amplifier. It is believed that the exponential output characteristic of the anti-log power amplifier is established on the general knowledge that the maximum value of the steering torque Ts decreases as the vehicle speed increases. The steering system disclosed in above U.S. Patent can produce suitable assistive steering torque thereby to give the driver a good steering feeling in all speed ranges from low to high vehicle speeds insofar as the steering speed Ns is relatively low.
With the above conventional steering system, however, the steering speed Ns at which the steering wheel is turned is not employed as a control parameter for the motor. Therefore, the rotational speed Nm of the motor may not be appropriately brought into conformity with the steering speed Ns at some values of the steering speed Ns. Moreover, where the steering speed Ns is relatively large, the value of the steering torque Ts becomes relatively large even if the vehicle speed is relatively high. When this happens, the motor generates a relatively large assistive torque which prevents the driver from gaining a normal road feeling.
The present invention has been made in view of the aforesaid problems of the conventional motor-driven power steering system for automotive vehicles.